Mannitol is by far the most abundant polyol in nature, occurring in bacteria, fungi, algae, lichens and vascular plants. See R. Bieleski, in: Encyclopedia of Plant Physiology, New Series Volume 13A. Plant Carbohydrates I. p.158-192 (1982); D. Lewis, in: Storage carbohydrates in vascular plants. Distribution, physiology and metabolism. p.158-179 (1984). Although several physiological roles have been proposed for mannitol, such as carbohydrate storage, regulation of carbon partitioning, osmoregulation and cofactor regulation, little information exists on the enzymatic pathways of mannitol synthesis and utilization. See R. Bieleski, supra; W. Loescher, Physiol. Plantarum 70:553-557 (1987); M. Rumpho et al., Plant Physiol. 73:869-873 (1983). Most information has come from studies of fungi and bacteria which contain a mannitol dehydrogenase that catalyzes the NAD-dependent or NADP dependent oxidation of D-mannitol to D-fructose. See, e.g., D. Quain et al., J. Gen. Microbiol. 133:1675-1684 (1987); W. Niehaus et al., Mycopathologia 107:131-137 (1989).
NAD-dependent mannitol-1-P dehydrogenase has been detected in bacteria, brown algae and fungi which catalyzes the conversion of mannitol-1-P to fructose-6-P. See, e.g., S. Horwitz et al., J. Biol. Chem. 239: 830-838 (1964); Richter and Kirst, Planta 170:528-534 (1987); R. Kiser et al., Arch. Biochem. Biophys. 211:613-621 (1981). All three enzymes are 2-oxidoreductases and can catalyze the reaction in either direction, toward synthesis or utilization of the polyol depending on the availability of oxidized or reduced cofactor and the pH.
Mannitol catabolism in higher plants is still poorly understood. Several labeling studies using [.sup.14 C]-mannitol suggest that mannitol is utilized at a slow rate in vascular plants. When celery leaf discs were incubated in [.sup.14 C]-mannitol, mannitol utilization was restricted to young leaf tissue. See J. Fellmann et al., Physiol. Plantarum 69:337-341 (1987). Suspension cultures of Daucus carota L. and Pinus radiata D. or carrot root tissue had a small uptake and metabolism of [.sup.14 C]-mannitol. See M. Thompson et al., Physiol. Plant. 67:365-369 (1986); W. Cram, Physiol. Plant. 61:396-404 (1984). Mannitol respiration, as measured by [1&lt;CO.sub.2 ] evolution, was monitored in fifteen higher plant species and ranged from very low rates (Avena sativa) to rates comparable to that of fructose or glucose (Fraxinus americana). White ash leaflets exposed to [.sup.14 C]-mannitol resulted in the formation of a small amount of [.sup.14 C]-fructose after 2 days, perhaps suggesting that the first step of mannitol utilization in this species may be oxidation to fructose. See P. Trip et al., Amer. Jour. Bot. 51:828-835 (1964). Although these labeling studies support the presence of a mannitol utilizing enzyme in higher plants, insofar as we are aware, no mannitol oxidizing enzyme has been isolated from a vascular plant.